The influence of higher environmental temperature (HET=30/-1 degrees C) on fentanyl-induced behavior was studied in unrestrained rats. Subacute exposure (3 days) of rats to HET significantly (P<0.01) increased the cataleptic effect of fentanyl citrate (0.5 mg/kg), in comparison to the corresponding exposure to normal environmental temperature (NET=22+/-1 degrees C). Also, the hyperthermic response of rats to a low dose of fentanyl citrate (0.2-0.5 mg/kg) was significantly (P<0.01) potentiated, and the hypothermic response to a high dose of fentanyl citrate (1.5 mg/kg) was significantly (P<0.05) attenuated after exposure to HET. Fentanyl-induced hyperexcitability, loss of righting reflex, loss of corneal reflex and analgesia were not significantly affected by HET. This study provides the first evidence on the influence of environmental temperature on drug-induced catalepsy. HET-induced potentiation of the cataleptic response to fentanyl could be the result of an interference with behavioral thermoregulation.